Home Page | About Me | Home Entertainment | Home Entertainment Blog | Politics | Australian Libertarian Society Blog | Disclosures

Mismatching fields on a PAL DVD ... or how tolerant are our eyes anyway?

This version: 10 July 2003, updated 3 May 2004 and 18 June 2006

Have a look at some of the reviews of the Region 4 versions of Independence Day and Mad Max. You will find them universally positive. Even the reviews of Pulp Fiction are mostly relieved that it wasn't as bad as the original release. Yet, incredibly, all three of these DVDs have been encoded, entirely or largely, in a way that potentially makes them look worse than NTSC transfers.

Are the various reviewers blind? Of course not. As it happens, the first two actually do look damned good on TV and with high quality projectors. But they are begging for trouble with some equipment.

So what is wrong with these DVDs?

PAL leaves NTSC for dead, especially for film

PAL has two significant advantages over NTSC. First, PAL offers higher vertical resolution (576 horizontal lines, rather than NTSC's 480). The second is that film sourced PAL DVDs do not suffer from interlacing problems. That's because PAL runs at 25 frames per second, so when films are transferred, they are merely sped up by four per cent, so that each PAL frame exactly matches each film frame (which run at 24 frames per second). NTSC, though, runs at 30 frames per second so somehow each four film frames need to be used to create five DVD frames. How this is done is explained here.

But what happens if an elementary mistake is made when transferring film to PAL DVD? That is what has happened with these three movies (and, who knows, maybe even more). So what is that mistake?

Interleaving frames

A film that you see at the cinema consists of a series of film frames run through the projector. Each frame is a little different to the previous one, and each pauses very briefly in the projector so that its image can be projected. This happens fast enough -- 24 frames in each second -- to fool our eyes and brains into thinking that these stuttering images show smoothly moving people and objects upon the screen.
Fig 1
Fig 2

When a film is transferred to DVD, a somewhat similar process occurs. But instead of a projector, the film runs through a telecine (a word formed from TELEvision and CINEma). Each frame is captured to magnetic tape in digital format, but not in quite the way you might expect. PAL TV (like NTSC) uses 'Interleaving'. So each frame is captured twice. The first capture consists of 288 horizontal lines, starting right at the top of the film frame and then dispersed evenly all the way to the just above the bottom. The next capture grabs exactly the same film frame, but starts just a little below the top line, and grabs the 288 lines in between the first set of 288 lines. Each capture is called a 'field'.

When this is duly transferred to a PAL DVD, each of these fields remains separate: the first consisting of all the odd-numbered horizontal lines from 1 to 575 (that adds up to 288 lines), the second consisting of all the even-numbered lines from 2 to 576.

When you are playing the DVD, the DVD player sends the first field, then the second field to constitute the frame. There are fifty fields sent every second to make 25 frames per second. All this works, once again, because the speed at which things proceed is sufficient to fool our eyes and brains into joining the two fields together to form a whole frame.

Now think of a vertical column on the original film frame. The field containing the odd-numbered lines, and the field containing the even-numbered lines will capture this column in the same position. All will line up nicely. If the column is moving, then the next frame will capture the column a little to one side of the original position. Of course, its two constituent fields will still show the column nice, straight and tall. No problems here at all.

But what if a mistake is made? What if the odd field is taken from one frame, and sent to the TV immediately followed by the even field from the next frame? Then all the odd lines on the TV will show the column in one place, and the even lines will show it in a different place.

That is what has happened with these three DVDs. So let's look at them individually.

Exhibit 1: Independence Day

The video transfer of Twentieth Century Fox's Independence Day to DVD must be well nigh perfect when you consider these comments: 'This [is] a gorgeously vibrant transfer throughout and it really shines', 'As for the DVD. Astounding. This is a damn fine looking disc and seems, what's the word I'm looking for?, complete!' But let's look closely at some frames.

In Figure 1 you can see three close details from two sequential frames taken from the movie (Title 2, Chapter 8, 11:40 from the Theatrical Version). They are of some White House drone's head, while the body on which it resides is walking down the hallowed halls. No, those aren't venetion blinds behind him. They are scan lines. You see that the odd and even fields in the frames of this section of the movies have been, incredibly, taken from different film frames.

In each case the same relative section of the frame was grabbed so you can see what's moving where with respect to the camera. In fact, the drone is walking from left to right with respect to the wall, but the camera is panning to the right even faster than he is moving, so leaving him slightly behind. Now do you see all those horizontal lines? They are bits of him and bits of the wall from different instants of time (indeed, filmed 1/24th of a second apart in time) because of the way the fields have been wrongly shuffled into inappropriate company in frames.
Fig 3

Now let's do a bit of work on these frames. With Photoshop it's pretty easy to remove every second line from any picture. Figure 2 shows four pictures which consists of the same frame detail as the first two frames above. But the first shows only the odd lines of the first frame, the second shows only the even lines of the same frame. Likewise for the second frame.

Notice the second and third fields? Clearly they belong in the same frame, but in fact come from the DVD from adjacent frames.

This mismatching of frames is not a defect of any particular DVD player. I captured these frames using PowerDVD, but the effect also appears on DVD players from Sony, Panasonic, Yamaha, Pioneer and Denon.

Interestingly, unlike the next two movies we shall look at, Independence Day does not have this problem all the way through. It appears in perhaps half of the movie, but there are plenty of scenes recorded properly. For example, most of the airfight scenes look okay. And the scenes featuring Will Smith at his girlfriend's house before he notices the spaceship are magnificently recorded.

NOTE: In mid 2004 20th Century Fox is releasing a remastered version of Independence Day. And this eliminates this field mismatch problem. Hurrah! See here for details.

Exhibit 2: Mad Max

That Australian classic Mad Max was released by Roadshow Entertainment but, sadly, it suffers precisely the same problem as the bad parts of Independence Day. Once again, nobody appears to have noticed this.

Here, in figure 3, I show a different aspect of the same problem. The top picture shows a frame containing a couple of wraiths floating over what might appear to be a restaurant. In fact, this is a hard cut between scenes in Title 1, Chapter 3, at 5:30. The odd lines of this shot show an interior shot of some people heading for the door. The even lines show an exterior shot of them coming out. I've deinterlaced this frame so you can see approximately what each frame should have looked like.
Fig 4

Exhibit 3: Pulp Fiction

Pulp Fiction has been released on DVD twice. The first was apparently so awful that Roadshow Entertainment had it re-mastered and re-released. This is from the second version.

Even in the slightly newer version, there are significant video problems quite aside from the misplaced fields. Notice particularly at the start when the venetian blinds flicker disastrously behind the characters.

But this movie does indeed also suffer from the problem of the DVD's frames being made from fields from adjacent film frames. Figure 4, for example, shows a detail from two adjacent frames of Uma Thurman's hand while she's in the retro food joint. The final detail is a fully reconstituted frame of how this shot should look. Unlike the others above, I haven't just removed one set of interlaced lines. I have actually deinterlaced the two adjacent DVD frames and reconstituted the film frame as it should have looked.

Why hasn't anyone noticed?

This is a good question. And it's hard to tell. But here are a number of possible reasons: So I think we can be largely thankful for our equipment. But the equipment should not have to do this. While a good projector, for example, can correct video sourced material virtually perfectly, this is impossible with this problem.


Given that almost no-one has noticed this, you may wonder why I have. That's simple. While I enjoy watching high quality reproduction of movies as much as anyone, I routinely set up my equipment to be as unforgiving as possible. Many good DVD players have a setting in their menus to show still pictures as either 'fields' or 'frames'. If set to the former, the DVD player will not show all the odd and even lines, but just one set of lines (it actually shows each line twice so that the screen is properly filled in). If set to 'frame' display, it will show both lines. I always set the DVD players I am using to 'frame' because I like to see if anything strange is going on.

When set to 'frame', any DVDs with these kinds of problems show rapid flickering between the two sets of fields.

Likewise, when I run PowerDVD, I leave it set to 'Force Weave' in the Advanced section of the Video Configuration panel. That's because I don't use PowerDVD to watch DVDs, but just to examine them.

Given how good movies like Independence Day and Mad Max look now, I have to wonder what they would look like if they were properly encoded on their DVDs.

UPDATE (18 June 2006): I've been thinking about this some more. First, if these movies did not suffer from this problem, then the interlacing strategy adopted by display devices would be weaving. But because a decent display detects the interlacing, it responds by employing either bobbing or some more advanced motion-adaptive deinterlacing strategy. Unfortunately, given the complexity of the motion in some of these movies, even motion-adaptive deinterlacing would tend to apply mostly bobbing to some scenes.

This means that the actual vertical resolution of these movies is halved during the problematic sections, since bobbing involves displaying only Field 1 (with the missing lines replaced by lines interpolated from those above and below each of them), then only Field 2 (ditto), then Field 1 from the next frame and so forth.

This totally eliminates the combing effect for most scenes, and the vertical resolution doesn't look as severely compromised as the above implies, simply because the interpolation does a close enough job most of the time.

But this probably is the explanation for the shimmering venetion blinds in Pulp Fiction. Consider, a sharply filmed venetion blind edge appears on, say, line 175 of the full frame. That is to say, line 174 and above are black, while line 175 and below are white. With the bobbing strategy in place, the odd field is displayed first. This makes line 173 black, line 175 white, and interpolates a new line 174 as grey (half way between the lines above and below it). Then the even field is displayed one fiftieth of a second later. This makes line 174 black, line 176 white, and interpolates a grey line 175. In other words, fifty times per second, line 174 is flickering between black and grey, and line 175 between white and grey.

Now, with Pulp Fiction, the venetion blinds aren't perfectly horizontal, but at a slight angle off the horizontal. This has the effect of leveraging a small vertical flicker into a large horizontal one. So I think that the field mismatch may, in fact, be the sole reason for the lousy picture quality in parts of Pulp Fiction.

© 2003-2006 by Stephen Dawson